Potato (Solanum tuberosum L.) is the fourth most important crop and the most important non-cereal food crop in the world. In potato cultivation, the major natural factor which limits yield is late blight caused by the oomycete pathogen Phytophthora infestans (Mont.) de Bary. This devastating disease can result in complete loss of crop yield unless controlled (Świeżyński and Zimnoch-Guzowska 2001). Fungicide treatment is currently the most common method to control late blight. However, the high cost of fungicide application is problematic, especially in developing countries. Moreover, because fungicide application can impact on health and environmental safety, the use of the chemicals is becoming restricted. In addition, the pathogen quickly evolves and some of the new variants are insensitive to commonly used fungicides (Day and Shattock 1997; Goodwin et at 1996). Therefore, the introduction of genetic resistance into cultivated potato is considered a valuable method to achieve durable resistance to late blight.
Two main types of resistance to late blight have been described in potato (Umaerus and Umaerus 1994). First, general resistance is often based on a major quantitative trait loci (QTL) and a few minor QTLs and results in partial resistance. Second, specific resistance is based on major dominant resistance (R) genes. In early breeding programs during the first half of last century, 11 R genes (R1-R11) derived from S. demissum were identified. Nine R genes, R3 (now separated as R3a and R3b) and R5-R11 were localized on chromosome 11 (Bradshaw et al. 2006; El-Kharbotly 1994, 1996; Huang et al. 2004; Huang 2005). Other R genes originating from S. demissum were mapped to different locations including R1 on chromosome 5 (El-Kharbotly et al. 1994; Leonards-Schippers et al. 1992) and R2 on chromosome 4 (Li et al., 1998). All R genes introgressed from S. demissum to cultivated potatoes have been overcome by the pathogen as new strains rapidly evolve that are virulent on the previously resistant hosts (Umaerus and Umaerus 1994). Consequently, partial resistance conferred by QTLs was thought to be more durable than resistance conferred by single R genes (Turkensteen 1993). However, partial resistance is strongly correlated with maturity type and makes resistance breeding more difficult (Wastie 1991). Also the genetic positions of QTLs often correspond to the region of R gene clusters (Gebhart and Valkonen 2001; Grube et al. 2000).
Hence, recent efforts to identify late blight resistance have focused on major R genes conferring broad-spectrum resistance derived from diverse wild Solanum species. Beside S. demissum, other wild Solanum species such as S. acaule, S. chacoense, S. berthaultii, S. brevidens, S. bulbocastanum, S. microdontum, S. sparsipilum, S. spegazzinii, S. stoloniferum, S. sucrense, S. toralapanum, S. vernei and S. verrucosum have been reported as new sources for resistance to late blight (reviewed by Jansky 2000; Hawkes 1990). To date, three R genes, RB/Rpi-blb1, Rpi-blb2 and Rpi-blb3 from S. bulbocastanum have been mapped on chromosome 8, 6 and 4, respectively (Nasess et al. 2000; Park et al. 2005a; van der Vossen et al. 2003, 2005). Another R gene, Rpi-abpt, probably from S. bulbocastanum, has been localized on chromosome 4 (Park et al. 2005b). Rpi1 from S. pinnatisectum on chromosome 7 (Kuhl et al. 2001), Rpi-mcq1 from S. mochiquense (Smilde et al. 2005) and Rpi-phu1 from S. phureja on chromosome 9 (Śliwka et al. 2006) have also been reported.
It is evident from a review of the existing art in this area that a significant need remains for novel genes, compositions and methods for conferring late blight resistance. In this patent disclosure, we meet this need by screening wild Solanum species and by cloning, and introducing and expressing novel Rpi resistance genes into potato.